SNP deputy leader Angus Robertson is to accuse the UK Government of running scared of a second independence referendum.
The party’s leader in Westminster will also accuse the Conservatives of having a “desperate desire” to prevent anyone rejecting “hard Brexit”.
It comes as Prime Minister Theresa May said it would it be “fundamentally unfair” to hold a second Scottish independence referendum in the next two years, as she prepares to outline plans for a more united UK.
Mr Robertson is expected to tell the SNP spring conference in Aberdeen that it is for the people of Scotland to decide the country’s future.
He will say: “It is clear from the PM’s panicked response to the Scottish Government’s decision to, rightly, give people in Scotland a choice over Scotland’s future, that the Tories are simply scared of the people’s choice.
“The Tories’ argument is not about process, it is about their desperate desire to prevent anyone having the chance to reject the hard right Brexit that they are so wedded to.
“The truth is it should not be for either Theresa May or the Scottish Government to decide Scotland’s future – that choice belongs to the parliament and the people of Scotland, and it is one this party will never shy away from.”
Deputy First Minister John Swinney has also issued a stark warning to Westminster that Scots must not be denied the “right to choose” the country’s future in a second independence referendum.
The Scottish Government minister insisted that the “kind of country we want to be” was at stake as the SNP administration in Edinburgh and the Conservative Government in Westminster appeared to be on a collision course over the constitution.
While First Minister Nicola Sturgeon has outlined plans to stage a fresh vote on independence between autumn 2018 and spring 2019, Mrs May intervened on Thursday to say “now is not the time” for such a ballot.
On Friday, the Prime Minister will pledge to forge a “more united” nation as she begins the Brexit process.
Mrs May will stress that the break from Brussels will require “one United Kingdom, all pulling together to get the best outcome”.
She will say: “The coming negotiations with the EU will be vital for everyone in the United Kingdom.
“Every person, every family, every business, every community the length and breadth of the United Kingdom – England, Scotland, Wales and Northern Ireland.”
It was “essential that we get the right deal” and that requires working together, she will say, adding: “That is what we have always done when faced with challenges.
“We have pulled together as one and succeeded together. “We are four nations, but at heart we are one people.
“And I will always ensure the voices and interests of England, Scotland, Wales and Northern Ireland are represented as we negotiate to leave the EU. “And I will always fight to strengthen and sustain this precious, precious union.”
Writing in The Times, the Prime Minister said it was important for the United Kingdom to “pull together”, stating that the SNP were trying to “force” her Government into agreeing to hold a vote before the country leaves the EU.
Mrs May said: “I do not agree with the SNP that now is the time to be talking about a second independence referendum.
“To do so now, while all our energies should be directed towards the negotiations with Europe, would make it more difficult to get the right deal for Scotland and the right deal for the UK as a whole.
“It is not something to which any responsible government could reasonably agree.”